The present invention is directed to automatic test equipment for electronic circuits. It is concerned particularly with the programmable power supplies used in such applications.
In the field of automatic test equipment, a common requirement is that the automatic test device apply a predetermined voltage at a terminal of the device under test. Typically, the tests to be carried out by the automatic equipment should be performed as fast as possible, so the predetermined voltage is applied quickly and, if possible, disconnected quickly.
In order to avoid damage both to the device under test and to the automatic test equipment, completion or interruption of the connection between the power supply and the device under test ordinarily should not be made while a significant voltage is present. If a significant voltage is present, arcing can occur, causing damage to the relay contacts that complete or interrupt the connection. Other transients can also occur, and these can be large enough to damage sensitive electronic components.
In the past, therefore, test sequences have typically included delays between the time when the power supply is turned off and the time when the contacts are opened. Such delays allow the voltage at the terminal to which the power supply is connected to die down. Although this usually is an effective way to avoid damage from transients due to disconnection while voltages are present, it can significantly reduce the speed with which the tests are performed.
In order to speed such tests, manufacturers of automatic test equipment have sometimes employed programmable power supplies of the four-quadrant type. This type of supply is capable of both supplying and drawing current, and it does whichever one is necessary to achieve the voltage to which it is set. For instance, if a four-quadrant power supply is set to 5 volts and is connected across, say, a capacitive load having a potential difference of 10 volts, the four-quadrant power supply draws current to hasten the change to the desired 5-volt level just as it would supply current if the load began at a zero-volt level. This type of supply is advantageous in automatic test equipment because, by setting the supply to zero volts just before it is disconnected from the device under test, it is possible to cause the voltage to reach zero in much less time than it would take if the voltage were merely permitted to die down.
Unfortunately, the cost of a four-quadrant programmable supply can be an order of magnitude higher than that of a simple fixed-voltage current-limited supply. Thus, although the four-quadrant supply affords the desired speed, it can significantly increase the cost of the test equipment.
It is accordingly an object of the present invention to achieve speed similar to that provided by the four-quadrant supply but without incurring the high cost that such supplies entail.